Acrylamide finds a wide variety of uses such as paper strengthening agent, coagulating or precipitating agent, and soil reforming agent. In recent years, it became possible to produce acrylamide in a comparatively easy manner according to a process wherein acrylonitrile is directly hydrated in the presence of a catalyst composed predominantly of metallic copper is place of the sulfuric acid method hitherto adopted.
In the above process wherein acrylonitrile is reacted with water in the presence of a copper-containing catalyst, the means disclosed, for example, in U.S. Pat. No. 3,642,894 for conducting the reaction while preventing the catalyst from contact with oxygen is generally employed to maintain the catalytic activity at a high level and to avoid side reactions.
This reaction is also disclosed, for example, in U.S. Pat. No. 3,911,009 which teaches a method with parameters for controlling the conversion rate of acrylonitrile to acrylamide at, for example, about 70%, and with the recovery of unreacted acrylonitrile by distillation or the like for re-use. The patent claims the method to be advantageous as a process for producing acrylamide on an industrial scale.
Means for recovering unreacted acrylonitrile from the reaction liquid obtained by the above mentioned general direct hydration process are known. For example, Japanese patent Prov. Publn. No. 7219/74 discloses a method wherein the reaction liquid is brought into contact with oxygen or an oxygen-containing gas to stabilize the acrylamide and thereafter water and acrylonitrile are evaporated off, leaving an aqueous solution of acrylamide having concentration of, for example, 30-50% by weight. Another known means for recovering acrylonitrile is disclosed in Japanese patent Prov. Publn. No. 36616/74 wherein the acrylonitrile and some water are distilled off in a non-oxidative atmosphere from the reaction liquid.
It has now been found that when the process wherein acrylonitrile and a part of the water are distilled off in a non-oxidative atmosphere from the reaction liquid is properly controlled, such process may be used to prevent satisfactorily the polymerization of the acrylamide, thus affording an easier operation than the process entailing the contact of the reaction liquid with oxygen or an oxygen-containing gas prior to distilling off the acrylonitrile.
The present invention relates therefore to an improvement in the known methods, the improved process comprising reacting acrylonitrile with water in the presence of a copper-containing catalyst while preventing the catalyst from contact with oxygen, distilling off unreacted acrylonitrile and a part of the water in a non-oxidative atmosphere from the reaction liquid to obtain an aqueous solution of acrylamide having a concentration of, for example, 30-50% by weight, and thereafter removing the impurities existing in the liquid such as copper and the like by a dual treatment of the solution with ion exchange resins.
As a means for removing the impurities from the watersoluble acrylamide obtained according to a process available before the development of the direct hydration process, there is known, for example, from the disclosure in U.S. Pat. No. 2,865,960 the use of a cation exchange resin activated in acidic form and an anion exchange resin activated in basic form.
On the other hand, a method is disclosed in Japanese patent Prov. Publn. No. 82011/75 wherein only a specific strongly basic anion exchange resin is used, and another method is disclosed in Japanese patent Prov. Publn. No. 83323/75 wherein the purification of a crude aqueous solution of acrylamide, obtained by the direct hydration process, is carried out by passing it through mixed beds of a cation exchange resin and an anion exchange resin.
According to applicants' study, it has been found that when only the treatment with a strongly basic anion exchange resin is carried out, it is difficult to remove copper ions and amine compounds to such a degree as to inhibit the production of polyacrylamide. Therefore, the joint use of a strongly acidic cation exchange resin and an anion exchange resin is considered to be necessary. However, when an aqueous solution of acrylamide is first passed through a strongly basic anion exchange resin and then through a strongly acidic cation exchange resin, the pH value of the aqueous solution of the acrylamide after the treatment with the strongly basic anion exchange resin increases to about 10 so that the solution of acrylamide becomes reactive and tends to form impurities by hydrolyzing and polymerizing reactions. On the other hand, when the method is reversed and the aqueous solution of acrylamide is first passed through a strongly acidic cation exchange resin and then through a strongly basic exchange resin, the pH value of the solution of acrylamide, after the initial lowering to 3-4, is again raised to about 10 by passing the solution to the strongly basic anion exchange resin whereby the solution easily undergoes hydrolyzing and polymerizing reaction to form impurities.
In a method where the aqueous solution of acrylamide is passed through mixed beds of a cation exchange resin and an anion exchange resin, the pH value of the aqueous solution of acrylamide at the outlet from the beds is almost neutral.
However, denaturation of the aqueous solution of acrylamide cannot be avoided as long as a strongly basic anion exchange resin is used. During operation of the mixed beds, the ion exchange resins undergo strong a fluidizing action by gas or liquid, upon regeneration of the resins and at the time of forming the beds, thus necessitating classification or mixing of the resins. As a result, the operation becomes complicated and is accompanied by such drawbacks as a significant loss of resins by impingement, crush and/or abrasion.
The aforementioned reveals that an aqueous solution of crude acrylamide cannot be purified into an aqueous solution of substantially high quality acrylamide merely by using a strongly acidic cation exchange resin and/or a strongly basic anion exchange resin in the process wherein unreacted acrylonitrile and a part of the water are distilled off from the reaction liquid substantially in the absence of oxygen. Further, it is often observed that, according to known conventional methods, an aqueous solution of acrylamide purified by the treatment with ion exchange resins changes in nature with the lapse of time; for example, within a period of one week to several months, changes are noticeable in the polymerization rate of acrylamide for producing polyacrylamide and in the solubility characteristics of polyacrylamide in water.